A black hole was a Ferengi drink that Jadzia Dax enjoyed having for breakfast. Dax was introduced to the black hole by a bartender she has known for 100 years. (DS9: "The Circle", "Playing God")
When Rom became a technician on Deep Space 9 in 2372, he told Quark that he had wiped his last table and mixed his last black hole. (DS9: "Bar Association")
The drink was ordered by another customer later that year. (DS9: "Hard Time")
Quark offered the exhausted Kira Nerys a frosty Bajoran ale or a black hole in 2373, but eventually brought her warm Enyak's milk to help get her sleep. (DS9: "Ties of Blood and Water") He also offers one to Rionoj in the opening scene of the episode The Homecoming.